Integrated circuit transformer devices for on-chip millimeter-wave applications

ABSTRACT

Methods are provided for building integrated circuit transformer devices having compact and optimized architectures for use in MMW (millimeter-wave) applications. The integrated circuit transformer devices have universal and scalable architectures that can be used as templates or building blocks for constructing various types of on-chip devices for millimeter-wave applications.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional Application of U.S. application Ser. No. 11/102,292filed on Apr. 8, 2005 now U.S. Pat. No. 7,427,801, the disclosure ofwhich is herein incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to integrated circuittransformer devices having compact and optimized architectures for usein MMW (millimeter-wave) applications. More specifically, the inventionrelates to integrated circuit transformer devices having universal andscalable architectures that can be used as templates or building blocksfor constructing various types of on-chip devices for millimeter-waveapplications.

BACKGROUND

In general, a transformer is a device that comprises a core (e.g.,magnetic), a primary coil and one or more secondary coils. The primarycoil receives electrical energy from a power source and couples theenergy to the secondary coil(s) by virtue of changing magnetic field,wherein the energy appears as an electromagnetic field across the coil.If a load is connected to the secondary coil, the energy is transferredto the load. The output power of the transformer cannot exceed the inputpower to the transformer, so the output current is reduced in directproportion to the gain in voltage (and vice versa).

RF (radio frequency) transformers are commonly used in electroniccircuits for applications such as impedance matching (for maximum powertransfer), AC voltage step-up/step-down, DC isolation between twocircuits, common mode rejection, filters, etc. In addition, atransformer can be used to construct a BALUN having, e.g., a balancedinput, where both input ports are isolated from ground to an unbalancedoutput where one output port is connected to ground.

For on-chip applications, transformers are typically constructed usingcoupled wires. A simple transformer structure comprises two wires withthe same windings on each side, which is referred to as a 1:1transformer or simply coupled-wires. By way of example, FIG. 1 is aperspective view of a semiconductor device having a conventionalintegrated transformer device. In FIG. 1, the semiconductor device (10)comprises a substrate (11) having integrated coplanar transformer (12)formed on a surface thereof. The transformer (12) comprises a firstconductor (13) (primary) and a second conductor (14) (secondary) thatare disposed parallel on the same layer. The conductors are patternedfrom a metal layer that is formed on the substrate surface, and thenencapsulated in a dielectric or insulating layer.

Transformer devices such as depicted in FIG. 1 typically have poorelectrical performance (e.g., low coupling, k=0.06) and exhibit highloss, especially when implemented for high frequency applications.Indeed, for lossy substrates such as silicon, the capacitive couplingbetween the metal lines (13), (14) and substrate (11) can result inincreased power dissipation. If the metal lines are reduced in width tolimit such capacitive coupling, the resistance of the metal lineincreases (e.g., via skin effect) resulting in increased powerdissipation. Moreover, conventional transformer designs such as in FIG.1 do not have well-defined return paths for closed environment EMconditions, which results in the electrical performance being moresensitive to surrounding metallic components. According, integratedcircuit coplanar transformer devices such as depicted in FIG. 1 aretypically used at lower frequencies where lower coupling factors, lossesdue to the skin effect, and inaccuracies caused by model to hardwarediscrepancies can be tolerated.

SUMMARY OF THE INVENTION

In general, exemplary embodiments of the invention include integratedcircuit transformer devices having compact and optimized architecturesfor use in MMW (millimeter-wave) applications. More specifically,exemplary embodiments of the invention include integrated circuittransformer devices having universal and scalable architectures that canbe used as templates or building blocks for constructing various typesof on-chip devices for millimeter-wave applications, including, forexample, power combiners, baluns, distributed active transformers, andfilters.

In one exemplary embodiment, an integrated circuit transformer includesa substrate, a ground shield formed on the substrate, a primaryconductor comprising an elongated conductive strip, and a secondaryconductor comprising an elongated conductive strip. The primary andsecondary conductors are aligned to form a coupled-wire structure thatis disposed adjacent to the ground shield. Moreover, the ground shieldcomprises a pattern of close-ended parallel elongated slots that aredisposed orthogonal to the primary and secondary conductors, as well asedge regions that provide current return paths that are collinear to theprimary and secondary conductors.

In one exemplary embodiment of the invention, the ground shield, theprimary conductor and the secondary conductor are disposed on differentlayers of an integrated circuit and aligned to form a stacked structure.A first insulation layer is disposed between the ground shield andsecondary conductor, and a second insulation layer is disposed betweenthe secondary conductor and the primary conductor. In one embodiment,the ground shield, and primary and secondary conductors are formed fromdifferent BEOL (back end of line) metallization layers.

An exemplary 1:1 integrated circuit transformer structure, which has aground shield design with orthogonal close-ended slots and edge regionsthat provide current return paths collinear to the primary and secondaryconductors, can be accurately modeled using 3D/2D compact modeling tobuild 1:1 transformer template models that are scalable by length. Morespecifically, the exemplary ground shield structure providesclosed-environment EM conditions which relaxes the parasitic effects andboundary conditions, thereby allowing compact models of 1:1 integratedcircuit transformer templates to be defined which are scalable by lengthindependent of the proximity of other structures in the layout.

Accordingly, an exemplary 1:1 integrated circuit transformer templatedevice can be accurately modeled using a lumped equivalent element model(e.g., RLC network) which is scalable by conductor length, Lc, (andother possible geometric parameters). The lumped equivalent elementmodel of an exemplary 1:1 integrated circuit transformer template devicecan be used in circuit simulation applications to obtain accuratesimulations of circuits having such 1:1 integrated circuit transformertemplates, irrespective of the circuit layout.

These and other exemplary embodiments, features and advantages of thepresent invention will be described or become apparent from thefollowing detailed description of exemplary embodiments, which is to beread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a conventional integrated circuittransformer device.

FIGS. 2A and 2B schematically illustrate an integrated circuittransformer device according to an exemplary embodiment of theinvention.

FIG. 2C schematically illustrates an integrated circuit transformerdevice according to another exemplary embodiment of the invention.

FIG. 2D schematically illustrates a ground shield pattern according toan exemplary embodiment of the invention.

FIG. 3 is a schematic circuit diagram of an integrated transformermatched power amplifier circuit according to an exemplary embodiment ofthe invention.

FIG. 4 is a schematic circuit diagram of an integrated power combinercircuit according to an exemplary embodiment of the invention.

FIG. 5 is an exemplary schematic circuit layout of the integrated powercombiner circuit of FIG. 4, according to an exemplary embodiment of theinvention.

FIG. 6 is an exemplary component icon that represents a lumpedequivalent model of an integrated circuit transformer device that isscalable by length, according to an exemplary embodiment of theinvention.

FIG. 7 is a flow diagram of a method for performing circuit simulationusing an integrated circuit transformer model that is scalable by lengthor other user specified parameters, according to an exemplary embodimentof the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIGS. 2A and 2B are diagrams that schematically illustrate an integratedtransformer device according to an exemplary embodiment of theinvention. In particular, FIG. 2A is a schematic side-view of asemiconductor device (20) comprising a substrate (21) having anintegrated transformer device (22) formed thereon, and FIG. 2B is aschematic top plan view of the exemplary device (20) of FIG. 2A. Asdepicted in FIGS. 2A and 2B, the transformer (22) comprises a groundelement (23) (or ground shield), a secondary conductor (24) and aprimary conductor (25), which are formed on separate layers of anintegrated circuit formed on the substrate (21) to form a stackedtransformer structure (22). The transformer (22) is encapsulated in aninsulative/dielectric material (26) (e.g., silicon dioxide or otherinsulative or dielectric materials), wherein a thin layer ofinsulative/dielectric material is disposed between the ground element(23) and secondary conductor (24) and between the secondary conductor(24) and the primary conductor (25). The exemplary embodiment depictedin FIGS. 2A and 2B shows the primary and secondary conductors (24) and(25) as being stacked coupled-wires having equal lengths Lc and widthsWc. Moreover, the primary conductor (25) has thickness denoted t₁ andthe secondary conductor has a thickness denoted t₂. As explained below,depending on the application, the primary conductor (25) can be formedthicker than the secondary conductor (24) to accommodate higher currentdensities (DC current).

FIG. 2B further illustrates a pattern for the ground shield element (23)according to an exemplary embodiment of the invention. The exemplaryground shield element (23) comprises a planar metal sheet that ispatterned to form a series of parallel close-ended slots (23 a) oflength Ls. The formation of the close-ended slots (23 a) results in theground shield element (23) having a series of parallel conductive strips(23 b) that are commonly connected at end portions thereof along edgeregions (23 c) (or “side-bar” elements) of the ground shield (23). Theclose-ended slots (23 a) and conductive strips (23 b) are disposed inrelation to the primary and secondary conductors (24) and (25) such thatthe slots (23 a) and strips (23 b) are disposed orthogonal to thedirections of wave propagation (as depicted by directional arrows for ACcurrent, I) along the length (longitudinal direction) of the stackedsecondary and primary conductors (24) and (25). The exemplary pattern ofalternating slots (23 a) and strips (23 b) extends along the entirelengths Lc of the stacked coupled-wires (24) and (25). Moreover, theside regions (23 c) (or side-bar elements) are designed to providecurrent return paths that extend collinear to the direction of wavepropagation lengthwise along the lengths of the stacked conductors (24)and (25).

It is to be appreciated that that an integrated transformer devicehaving stacked primary conductor, secondary conductor, and slottedground shield framework, such as depicted in FIGS. 2A and 2B provides acompact 1:1 integrated circuit transformer structure that is highlyefficient and provides high-coupling factors (e.g., k=0.8 or better) foruse in millimeter waver applications. Indeed, a stacked coupled-wireframework (as compared to conventional coplanar structures of FIG. 1)confines the EM fields between the primary and secondary conductors,thereby yielding increased coupling between the primary and secondaryconductors and lower losses due to less current crowding effect (lessskin effect by width), as well as reduced frequency dependence whichallows for higher bandwidth modeling.

Moreover, the slotted ground shield framework, which has a series ofdensely-spaced conductive strips (23 b) and close-ended slots (23 a)arranged orthogonal to wave propagation along the primary and secondaryconductors, provides effective EM shielding from the substrate (e.g.,silicon substrate) which significantly reduces substrate induced losses,and enhances the coupling factor, k, between the primary and secondaryconductors. More specifically, the conductive strips (23 b) providesmeans for preventing EM energy from flowing into the substrate (21).Furthermore, the use of close-ended slots (23 a) arranged orthogonal towave propagation along the stacked conductors (24) and (25) providesmeans for reducing the generation of image currents flowing in theground shield the direction of wave propagation, thereby decreasing thecoupling between the conductors (24) and (25) and the ground shield (23)and effectively enhancing the coupling factor between the primary andsecondary conductors (24) and (25).

Furthermore, the edge regions (23 c) (or side-bar elements) of theground shield (23) provide well-defined current return paths (for closedenvironment EM conditions) that are collinear to wave propagation. Inaddition, the edge regions (23 c) provide means for shielding/isolatingthe transformer structure from the effects of other components/elements(e.g., vias, amplifiers, wiring, grounds, etc.) surrounding or inproximity to the transformer.

In fact, as will be explained below, a slotted ground shield withside-bar elements enables highly compact modeling of the integratedcircuit transformer structure to build a lumped equivalent model (e.g.,RLC elements), which is scalable by length, Lc. The lumped equivalentmodel for an integrated transformed device enables circuit/networkanalysis of an integrated circuit having one or more of the transformertemplates without requiring a full 3D simulation of the integratedcircuit. In this regard, it is to be further appreciated that anintegrated circuit 1:1 transformer device such as depicted in FIGS. 2Aand 2B can be readily used as a template (or building block), which isparameterized by length, for constructing various integrated circuitdevices and modular structures including, but not limited to, poweramplifiers, n:1 impendence transformers, and power combiners. Exemplaryembodiments of integrated circuit devices that implement 1:1 integratedtransformer templates will be described below with reference to FIGS.3-5, for example.

Furthermore, an IC transformer device such as depicted in FIGS. 2A and2B can be used as a resonant tuning circuit to provide a high-efficiencyresonating structure, e.g., using simple capacitances at both ends ofthe secondary conductor, or by designing the conductor length Lc equalto ½ wavelength for the fundamental operating frequency. The ICtransformer structure provides a strong peak in current coupling, andenables increased voltage coupling as compared to coupling obtained dueto tank resonance circuits. Moreover, the IC transformer structureprovides a strong peak in power coupling and provides a real (ratherthan reactive) input impedance. Moreover, an integrated circuittransformer can be designed with a symmetrical framework to provideequal input and output impedances (e.g., designing parallel,evenly-spaced close-ended slots in ground shield).

It is to be appreciated that various semiconductor fabrication methodsmay be used for constructing an IC transformer device as depicted inFIGS. 2A and 2B. For high-frequency applications, the exemplary ICtransformer device can be fabricated based on compound semiconductortechnologies such as GaAs (gallium-arsenide) or SiGe (silicongermanium), or other technologies that are suitable for highlyintegrated circuits operating in millimeter-wave frequency ranges.

In one exemplary embodiment, an IC transformer device may be fabricatedby forming the primary and secondary conductors and ground shield fromdifferent metallization layers that are deposited and processed using aBEOL (back-end-of-line) process. More specifically, in one exemplaryembodiment, after forming active circuit regions on a substrate, aninsulation layer (e.g., silicon dioxide (SiO2)) insulating layer andfirst metallization layer can be sequentially formed over the activeregions. The ground shield (23) can be formed by patterning a portion ofthe first metallization layer. Following the first metallization, asecond insulation layer and second metal layer are sequentially formed.The portion of the second metal layer that is aligned to the groundshield (23) is patterned to form the secondary conductor (24).Thereafter, a third insulation layer and third metallization layer aresequentially deposited, and the portion of the metal layer aligned tothe secondary conductor (24) is patterned to form the primary conductor(25). In another exemplary embodiment of the invention, the aboveprocess can be reversed to form a stacked IC transformer with the groundshield disposed over the secondary and primary conductors. In suchembodiments, it is preferable that the metal layer forming the primaryconductor is of sufficient thickness to handle increased currentdensities, for certain applications such as described below.

In one specific exemplary embodiment, the stacked IC transformer can beformed using the current SiGe8HP semiconductor technology ofInternational Business Machines Corporation, wherein the BEOL processincludes five layers of metallization formed over the active chipsurface. With such technology, the top three layers of metals can beutilized for the primary, second and ground shield elements, and thelower two layers are utilized to form connections between activecomponents and form connections from active components to components ofthe transformer elements.

It is to be appreciated that the dimensions of an IC transformer deviceas depicted in FIGS. 2A and 2B will vary depending on the applicationand semiconductor processing technologies implemented. For instance, theslot length Ls should be of sufficient length to extend past the sidesof the conductors (24) and (25) to prevent formation of longitudinalimage currents on the ground shield due to coupling of the EM fieldsbetween the secondary and ground elements. Moreover, the width andspacing of the slots (23 a) and strips (23 b) should be sufficientlysmaller than the operating wavelength to avoid formation of longitudinalimage currents on the conductive strips (23 b). Moreover, it ispreferable to provide a dense pattern of conductive strips (23 b) tominimize capacitive coupling between the substrate and secondaryconductor. In practice, the slot/strip widths are limited by the designrules of the given fabrication process.

Moreover, the distances/separation between the primary and secondaryconductors should be sufficiently smaller than the operating wavelength.Moreover, the conductor widths WC will be selected based on factors suchas the operating frequency and resistivity of the material used to formthe primary and secondary conductors, to thereby provide sufficientsurface area to minimize resistive losses resulting from skin effectphenomenon. In one exemplary embodiment of the invention, the secondaryconductor can be formed wider than the primary conductor to provideincrease coupling of fringing fields from the primary conductor. Thethickness t1 and t2 of the conductors will be determined based on thefabrication methods implemented, and/or can be selected to accommodatethe current densities for the given application.

In other exemplary embodiments of the invention, coupled primary (25)and secondary (24) conductors can be stacked horizontally (disposedadjacent to each other) over the ground shield (23), such as depicted inFIG. 2C. In effect, the resulting structure can be viewed as thecoplanar conductors (13) and (14) as depicted in FIG. 1, except that theconductors (24) and (25) in FIG. 2C are much thicker than wider. In thismanner, increased coupling between the conductors (24) and (25) may beachieved, while forming the conductors (24) and (25) on the same layerof an integrated circuit.

In yet other exemplary embodiments of the invention, the edge regions ofthe ground shield can have longitudinal slots formed therein, whereinthe slots are formed collinear to wave propagation. For instance, FIG.2D is an exemplary ground shield 23′ which is similar to the groundshield (23) of FIG. 2B, except that the side edge regions havelongitudinal slots (23 d) that form conductive strips (23 e) which areconnected to the ground shield structure. The slotted edge regionsprovide increased isolation from nearby objects and prevent coupling ofeddy currents flowing on the substrate.

FIG. 3 schematically illustrates an integrated circuit power amplifierwhich is constructed using an integrated circuit transformer device,according to an exemplary embodiment of the invention. Morespecifically, FIG. 3 depicts an exemplary integrated circuit poweramplifier (30) for use with MMW applications, which implements anintegrated circuit transformer (33) for high-efficient coupling ofoutput power. In general, the power amplifier (30) comprises an inputmatching network (31) connected to the differential inputs of a cascodedifferential amplifier (32), an integrated transformer (33) connected tothe differential output nodes of the differential amplifier (32) and anoutput matching network (34). The input matching circuit (31) matchesthe differential inputs, V_(in), to a desired characteristic impedance(e.g., 50 Ohms) using series inductive microstrip lines (31 a) andparallel capacitors (31 b). The output matching circuit (34) matches thedifferential outputs, V_(out), to a desired characteristic impedance (50Ohms) using series inductive microstrip lines (34 a) and parallelcapacitors (34 b).

The cascode differential amplifier (32) includes a first cascode stagecomprising serially connected transistors T1 and T3, and a secondcascode stage comprising serially connected transistors T2 and T4. Thetransistors T3 and T4 (common-emitter input stages) drive respectivetransistors T1 and T2 (common-base output stages). A bias Voltage,Vbias, is applied to the base terminals of transistors T3 and T4 and aconstant DC voltage, Vgbs, is applied to the base terminals oftransistors T1 and T2. These bias voltages are selected to prevent thebase to collector voltage across each transistor T1˜T4 from exceedingthe breakdown voltage of the transistor in the cascode array and todivide the peak voltage applied between the collectors of T1 and T2 andemitters of respective transistors T3 and T4 to prevent breakdown.

In the exemplary embodiment of FIG. 3, the integrated circuittransformer (33) is schematically depicted as having a primary conductor(33 a) with input terminals (nodes N1 and N2) and a secondary conductor(33 b) with output terminals (nodes N1′ and N2′). The collectorterminals of transistors T1 and T2 are connected to nodes N1 and N2,respectively. In one exemplary embodiment, the integrated circuittransformer (33) comprises a 1:1 IC transformer having a framework asdescribed in FIGS. 2A and 2B, wherein the primary conductor (33 a) andsecondary conductor (33 b) are stacked over a slotted ground shield. Inthis embodiment, the collectors of transistors T1 and T2 (output nodesN1, N2) are connected to the length ends of the primary conductor (33 a)of the integrated transformer (33), and bias voltage VCC connection ismade to a center region between the length ends of the primary conductor(33 a).

With the exemplary framework of FIG. 3, the 1:1 IC transformer (33)couples the differential output voltage (VN1-VN2) of the differentialamplifier (32) to the secondary conductor (33 b) at nodes N1′ and N2′.The 1:1 IC transformer (33) is utilized as a resonator to providehigh-efficient coupling of the output power of the amplifier. In anotherexemplary embodiment, a 1:1 IC transformer can be coupled to the inputof the differential amplifier circuit.

Moreover, in the exemplary embodiment of FIG. 3, the input terminal,Vin, are driven differentially with equal amplitude, opposite phase, RFsignals, which creates a virtual AC ground on the primary conductor (33a) at the point where the VCC supply connection is made, and at theground G connection between the sources of the transistors T3 and T4.The virtual AC grounds are points of low loss and low impedance. Assuch, the virtual AC ground at the VCC connection point eliminates theneed for a separate choke inductor and/or a large on-chip bypasscapacitor at the VCC supply connection.

It is to be appreciated that a plurality of 1:1 IC transformer devicescan be used as templates or building blocks for constructing other ICcomponent/devices for MMW applications. For example, FIG. 4 is aschematic circuit diagram illustrating an integrated power combinercircuit according to an exemplary embodiment of the invention. Morespecifically, FIG. 4 schematically illustrates a power combiner circuit(40) that is implemented using four transformer template devices (41,42, 43 and 44), and a plurality of differential push-pull amplifiers(45, 46, 47 and 48), which are arranged in a square or circulargeometry. Each transformer template device (41, 42, 43 and 44) isschematically depicted as having a primary conductor (41 a, 42 a, 43 aand 44 a), a secondary conductor (41 b, 42 b, 43 b and 44 b) and aslotted ground shield (41 c, 42 c, 43 c and 44 c), respectively, similarto the exemplary IC transformer framework of FIGS. 2A and 2B.

In the exemplary embodiment, the secondary conductors are connected inseries and the secondary conductor (41 b) of transformer (41) isseparated at a mid section thereof to provide differential +− outputterminals, Vout. Moreover, each primary conductor (41 a, 42 a, 43 a, and44 a) is connected between differential outputs of adjacent poweramplifiers and has a center point that is connected to VCC terminal toprovide DC bias voltage to the amplifiers. The power combiner circuit(40) can be constructed using 4 push-pull amplifiers each with a 1:1transformer (such as depicted in FIG. 3), whose secondary conductors areconnected in series. With such exemplary embodiment, an input voltage,Vin, is divided into four equal amplitude, equal phase signals, whichare simultaneously applied to different ones of + differential inputs ofthe amplifiers (45, 46, 47 and 48), and an inverted input voltage, Vin,is divided into four equal amplitude, equal phase signals, which aresimultaneously applied to different ones of − differential inputs of theamplifiers (45, 46, 47 and 48), to thereby create an alternating currenton each primary conductor. The AC currents on the primary conductors arecoupled to the serially connected secondary conductors to generatecombined output voltage, Vout. With the exemplary embodiment, since theprimary conductors are independent, the distributed architectureprovides a power combiner as well as a 4:1 impedance transformer. Inother words, the exemplary power combiner (40) provides combining andimpedance transformation for power amplifier applications.

FIG. 5 schematically illustrates an exemplary circuit layout (50) of thepower combiner circuit of FIG. 4, according to an exemplary embodimentof the invention. More specifically, FIG. 5 schematically illustrates anexemplary layout pattern (50) comprising a ground shield (51), biasvoltage VCC network (52), a continuous secondary conductor (53),connectors (54) and exemplary embodiments for the push-pull amplifiers(45, 46, 47 and 48). The exemplary ground shield (51) pattern integrallycombines each of the separate ground shields (42 c, 43 c, 44 c and 45 c)of the transformer template devices (42, 43, 44 and 45) depicted in FIG.4. The ground shield (51) is patterned to form separate patterns ofclose-ended slots that extend orthogonally to the respective transformerconductors.

The distributed transformer framework comprises the continuous secondaryconductor (53) having differential output terminals Vout, and separateprimary conductors (41 a, 42 a, 43 a, 44 a) that are stacked over, andaligned to, different portions along the length of the secondaryconductor (53). The DC supply voltage network (52) comprises a pluralityof wires that connect to center portions of each of the primaryconductors (41 a, 42 a, 43 a, 44 a).

FIG. 5 further depicts each of the push-pull amplifiers (45), (46), (47)and (48) implemented using respective pairs of common-emitter gainstages (45 a/45 b), (46 a/46 b), (47 a/47 b), (48 a/48 b) having BJT(bipolar junction transistors). It is to be understood that thedifferential push-pull amplifier frameworks depicted in FIG. 5 aremerely exemplary, and that other differential push-pull amplifierdesigns, such as depicted in FIG. 3, may be implemented. Moreover, theamplifiers can be implemented using CMOS devices. Each primary conductor(41 a, 42 a, 43 a, 44 a) has angled length end portions that areconnected to differential outputs of respective push-pull amplifiers viaconnectors (54).

In the exemplary embodiment, the differential amplifiers (45, 46, 47 and48) are connected to the primary conductors (42 a, 43 a, 44 a, 45 a)such that each primary conductor (42 a, 43 a, 44 a, 45 a) has acorresponding push/pull amplifier pair, i.e., amplifier pair (48 a/45 b)for primary conductor (41 a), amplifier pair (45 a/46 b) for primaryconductor (42 a), amplifier pair (46 a/47 b) for primary conductor (43a) and amplifier pair (47 a/48 b) for primary conductor (44 a). Theprimary conductors provide low resistance paths for the dc current toflow from the DC supply to the drains of the transistors. The sourceterminals of adjacent transistors of adjacent amplifiers are connectedtogether and share a common ground. As shown by the “+” and “−” symbols,each pair of adjacent transistors are driven in opposite phases.

A power distribution network (not shown) is used to apply synchronizeddifferential power to the gates of the transistors. For example, in theexemplary embodiment of FIGS. 4 and 5, two sets of three Wilkinson powerdividers can be implemented to divide both an input signal Vin andinverted input signal/Vin into four equal phase, equal amplitude, inputsignals that are synchronously applied to the different inputs ofamplifiers 45, 46, 47 and 48. The exemplary power combiner circuit inFIG. 5 achieves power combining by operating the push-pull amplifiers toconduct identical synchronized ac currents at the fundamental frequencyon respective primary conductors, thereby inducing a corresponding accurrent in the continuous secondary conductor. The secondary conductor(53) harnesses the induced AC power to generate a voltage at terminalsVout that is equal to the sum of the differential voltages of the 4push-pull amplifiers.

In another exemplary embodiment of the invention, the circuits of FIGS.4 and 5 can be extended to construct a power splitter. In such instance,the continuous secondary conductor in FIG. 5 would be the primaryconductor that receives an input signal (at the Vout terminal) and eachseparate primary conductor in FIG. 5 would be the secondary conductorsthat each couple about ¼ of the EM energy of the primary. In thismanner, a symmetrical 1:4 power splitter could be implemented.

It is to be appreciated that an integrated transformer device accordingto an exemplary embodiment of the invention can be accurately modeledusing lumped equivalent elements to define models that are scalable byconductor length, Lc, and other possible geometric parameters of theintegrated transformer. More specifically, the exemplary 1:1 transformerstructure having a ground shield design with orthogonal slots andcollinear side bars as described above with reference to FIGS. 2A and2B, for example, enables compact modeling of the integrated transformerstructure by providing closed environment EM conditions which relaxesthe parasitic effects and boundary conditions. This provides the abilityto define compact models of 1:1 transformer templates that are scalableby length independent of the proximity of other structures in thelayout.

Currently, various applications are commercially available that can beused to build and simulate various types of 2D/3D integrated circuits,components and other structures for microwave and RF applications. Theseapplications allow designers to build 3D/2D integrated circuit layoutswith interconnects and apply full-wave 3D circuit EM simulators thatdetermine the electrical properties and response of integrated circuitdesigns taking into account the effects of other transformers, wires,interconnects or other radiating structures, which provide undesirableimpedances (parasitic capacitances and inductances) that, in practice,affect circuit performance. Some applications include methods forextracting a two port frequency dependent network representation ofvarious circuit components using the 3D processing results and create alumped equivalent circuit for such components. However, when the 3Dcircuit layout is modified, the lumped equivalent circuit no longeraccurately defines the characteristics/properties of the modified 3Dcircuit, and a full 3D EM simulation must be performed again to extractrelevant parameters to recreate the lumped equivalent circuits forcircuit simulation using simple nodal analysis, for example.

It is to be appreciated, however, that an exemplary 1:1 transformerstructure with a slotted ground shield design with side-bars allows thetransformer structure analyzed in isolation using circuit simulationwithout having to perform a full wave 3D analysis to consider theeffects of metal objects (on-chip transmission lines, inductors, bondpads, interconnects etc.) in proximity to the integrated transformertemplate. In this manner, compact models of a 1:1 transformer structurecan be accurately defined with a lumped equivalent model which can beuniversally applied for circuit simulation. For example, the schematicpower combiner circuit (40) can be accurately simulated using lumpedequivalent models of a 1:1 transformer template structure according toan embodiment of the invention for each of the schematically depictedtransformers (41, 42, 43 and 44), irrespective of the 3D configurationand circuit layout (e.g., FIG. 5). In this regard, the transformermodels are decoupled from each other and other circuit components,allowing the circuit to be simulated using simple nodal analysis withlumped equivalent models. This is in contrast to conventional integratedcircuit coupled line transformers (such as depicted in FIG. 1) whichrequire 3D EM simulations for different layouts and circuitconfigurations in MMW applications.

In one exemplary embodiment described below, an integrated circuit 1:1transformer device having a framework as depicted in FIGS. 2A and 2B canbe modeled using a simple RLC filter network (including coupledinductors and dependent sources) for circuit simulation applications. Acompact model of an integrated circuit transformer device can accuratelydescribe the electrical properties/characteristics of the integratedcircuit transformer in all modes of operation and all secondary/primarycomplex loads, and such properties up to third harmonic of a fundamentalmillimeter wave frequency. The lumped equivalent model can be used toreadily determine network values by simple 2D EM simulations.

In one exemplary embodiment of the invention, a simulation model for theexemplary stacked 1:1 transformer template with slotted ground shield,which is scalable by length, can be built as follows. Initially, a 1:1integrated circuit transformer template structure can be designed usinga 3D circuit layout tool. Thereafter, 3D EM solver methods are appliedto extract the parameters of the transformer structure per unit length.In particular, a 2×2 capacitance matrix can be computed for the twosignal lines (primary and secondary conductors) above the ground plane(consisting of two collinear side bars and orthogonal conductive stripsusing a 3D static capacitance solver. The side-bar structures enable useof a 3D full wave emulation of the transformer template to extract themodel under the assumption of closed EM environment. It is to beappreciated, however, that a 2D solver could be used as a goodapproximation when the pattern of orthogonal stripes is sufficientlydense (which is preferred to shield against silicon substrate losses).Moreover, a static solver is sufficient to determine the capacitanceparameters, when the separation between the metals is negligible ascompared with the wavelength.

Thereafter, a frequency dependent 2×2 inductance matrix and frequencydependent 2×2 resistance matrix is obtained using a 2D simulation of areduced transformer structure which contains the two signal lines and areduced ground plane containing only the two collinear side bars (returnpath lines) without the orthogonal stripes. Analysis using the reducedstructure is justified since the Z element of the model describes onlythe distribution of the longitudinal currents. The minimal bandwidthrequired for these EM simulations is from the fundamental MMW frequencyup to the third harmonics. A full solution for the skin and proximityeffects is performed due to the millimeter wave frequencies.

Then, using the EM solver data, an RLC network is constructed for twocoupled lines above a ground plane. The RLC network comprises anappropriate filter network (ladder circuit) which describes thefrequency dependence of the resistance and inductance matrix in thegiven bandwidth. In addition to the RLC passive elements, and the mutualinductance coupling elements between the inductors, dependent sourcesare also included to correctly describe the frequency dependence in boththe even and odd modes. These dependent sources are connected in amanner which assures the model passivity (unconditional stability).Several of such RLC segments are used for achieving a distributed model(e.g., at least 10 segments are used per wavelength).

Accordingly, exemplary model building methods described above can beused to define lumped equivalent models for 1:1 integrated transformertemplate devices which are scalable by length (or other geometricparameters) and which can be universally applied for circuit design andsimulation. Indeed, various compact models for transformer templatedevices can be defined, for structures having different geometricparameters such as slot distances, slot widths, conductor widths andthicknesses, for example, and for structures that are formed usingdifferent semiconductor fabrication technologies, for example.

With such methods, one or more transformer models can be included aspart of a design kit or library of models in circuit simulationapplication to enable design and simulation of integrated circuits forMMW application which implement exemplary transformer template devicesaccording to the invention. The models can be built allowingparameterization with user-defined parameters (e.g., conductor length,etc.) can be specified for simulations. FIG. 6 depicts an exemplary ICONrepresentation for a 1:1 transformer template model that can be used ina simulation application. The icon of FIG. 6 is a schematic depiction ofa transformer (60) having a primary conductor (61) and secondaryconductor (62) over a ground plane (63), for a 1:1 transformer templatemodel that allows values for the conductor length L in um to beuser-defined.

FIG. 7 is a flow diagram of a method for performing circuit simulationusing a lumped equivalent model of an integrated circuit transformerthat is scalable by length or other user specified parameters, accordingto an exemplary embodiment of the invention. The method of FIG. 7 can beimplemented in any suitable 3D/2D design and simulation applicationhaving user interface that enables a user to build schematic circuits,for example. Referring to FIG. 7, a user can build schematic circuitusing device/component icons such as the transformer template icon ofFIG. 6 (step 70). In such instance, the application would have a libraryof models for each of the components. As noted above, according to oneexemplary embodiment of the invention, the user would be able to selectone or more types of transformer template models (which are provided aspart of the library), in which one or more geometric parameters, such aslength, would be a variable defined by the user (step 71). Depending onthe model design, other geometric parameters such as slab width, etc.could be variables input by the user.

After the schematic circuit is created, the user could perform a circuitsimulation by specifying values for one or more parameters (e.g.,voltages, frequency ranges, device values, etc.) of lumped equivalentmodels that enable input of user-specified parameters, and thenexecuting the appropriated commands to obtain the desired electricalresponse/characteristics of the circuit based on the user specifiedparameters (step 72). The circuit simulation is performed by performinga nodal analysis of the given circuit using the lumped models for theconstituent circuit components/devices as parameterized by theuser-specified parameters (step 73). Thereafter, the results of thecircuit simulation would be presented to the user for review (step 74).

It is to be appreciated that the exemplary model building and circuitsimulation methods described above are merely exemplary. In oneembodiment, a circuit design and simulation tool can include a libraryof one or more predefined transformer template device models withscalable parameters, which can be used for circuit analysis. In otherexemplary embodiments of the invention, 2D/3D circuit design andsimulation tools can be extended to include the transformer templatemodel building methods described above, which allow a user to createcustom, user-defined compact transformer models by generating customtransformer template structures, and then apply relevant model buildingmethod to extract the 2D/3D EM parameters needed to construct acorresponding lumped equivalent model. In other embodiments, 2D/3D toolscan be implemented with both library of predefined transformer templatemodels, and the tools needed for generating custom transformer templatemodels.

Although illustrative embodiments have been described herein withreference to the accompanying drawings, it is to be understood that thepresent invention is not limited to those precise embodiments, and thatvarious other changes and modifications may be affected therein by oneskilled in the art without departing from the scope or spirit of theinvention. All such changes and modifications are intended to beincluded within the scope of the invention as defined by the appendedclaims.

We claim:
 1. A method of fabricating a semiconductor device, comprising:providing a substrate having a substrate surface in a lateral plane;forming a layer of conductive material on the substrate surface;patterning the layer of conductive material to form a ground shieldcomprising a plurality of close-ended parallel elongated slots extendingin a first direction and edge regions having a plurality of longitudinalslots extending in a second direction substantially perpendicular to thefirst direction and disposed adjacent to ends of the close-endedparallel elongated slots; and forming a primary conductor and asecondary conductor disposed above the ground shield, wherein theprimary conductor and the secondary conductor lie in respectivedifferent planes and have substantially parallel longitudinal axesextending in a first direction perpendicular to the lateral plane,wherein the primary conductor and the secondary conductor form acoupled-wire structure.
 2. The method of claim 1, wherein the groundshield, primary conductor and secondary conductor are formed ondifferent layers of an integrated circuit to form a stacked transformerdevice.
 3. The method of claim 2, wherein the ground shield, the primaryconductor and secondary conductor are formed from differentmetallization layers using a back end of line process.
 4. The method ofclaim 1, further comprising encapsulating the coupled-wire structure ina dielectric material.